Without limiting the scope of the present invention, its background will be described with reference to perforating a hydrocarbon bearing subterranean formation with a shaped charge perforating gun apparatus, as an example. After drilling a section of a subterranean wellbore that traverses a hydrocarbon bearing subterranean formation, individual lengths of metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path through which fluids from the formation may be produced to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string or to place addition cement behind the casing string, hydraulic openings or perforations must be made through the casing string and a distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges located within one or more perforating guns that are deployed within the casing string to a position adjacent to the desired location. Conventionally, the perforating guns are formed from a closed, fluid-tight hollow carrier gun body adapted to be lowered into the wellbore on a conveyance such as wireline, coiled tubing, jointed tubing or the like. Disposed within the hollow carrier gun body is a charge holder that supports and positions the shaped charges in a selected spatial distribution. The shaped charges have conically constrained explosive material therein. A detonating cord that is used to detonate the shaped charges is positioned adjacent to the initiation ends of the shaped charges. The detonating cord can be activated electronically or mechanically when it is desired to firing the perforating guns.
In such closed, fluid-tight type gun bodies, the explosive jets produced upon detonation of the shaped charges must penetrate the hollow carrier gun body before penetrating the casing wall of the wellbore and the adjacent formation. To reduce the resistance produced by the hollow carrier gun body and increase the depth of perforation penetration into the formation, the perforating gun body may be provided with scallops or other radially reduced sections at the target regions through which the explosive jets pass. As such, the scallops in the hollow carrier gun body must be positioned in a spatial distribution that aligns with or corresponds to the spatial distribution of the shaped charges held within the gun body by the charge holder.
Once the perforating guns are deployed to the desired location, firing a conventional perforating gun results in perforations into the formation having substantially the same depth and entry hole dimensions. It has been found, however, that in certain operations, it may be desirable to generate perforations that do not have substantially the same depth and entry hole dimensions. A need has therefore arisen for a perforating gun apparatus that is operable to generate perforations having variable penetration profiles.